Stockholders' Equity (Details) (USD $)	0 Months Ended					12 Months Ended
	Nov. 08, 2012	Oct. 31, 2012	Jul. 31, 2012	Apr. 30, 2012	Jan. 27, 2012	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Class of Stock [Line Items]
Common stock, shares outstanding						151,601,209	148,107,611
At the market stock offering program, shares issued						151,680,109	148,186,511
At the market stock offering program, net proceeds						$ 247,027,000	$ 439,037,000	$ 0
Shares of Common Stock upon the exercise of options to purchase Common Stock by certain employees						22,823	316,159	638,957
Shares of its Common Stock in connection with the redemption of an equal number of OP Units						1,110,660	2,919,323
Dividend paid on common stock (in dollars per share)		$ 0.55	$ 0.55	$ 0.55	$ 0.55
Common Stock, Dividends, Per Share, Declared	$ 0.65
Atm Program [Member]
Class of Stock [Line Items]
At the market stock offering program, aggregate value of common stock						600,000,000
At the market stock offering program, shares issued						2,347,500
At the market stock offering program, gross proceeds						249,800,000
At the market stock offering program, net proceeds						247,000,000
At the market stock offering program remaining amount available for issuance						$ 305,300,000
Quarter 4 2011 [Member]
Class of Stock [Line Items]
Dividend payable date						Jan. 27, 2012
Dividend shareholder of record date						Dec. 31, 2011
Quarter 1 [Member]
Class of Stock [Line Items]
Dividend payable date						Apr. 30, 2012
Dividend shareholder of record date						Mar. 30, 2012
Quarter 2 [Member]
Class of Stock [Line Items]
Dividend payable date						Jul. 31, 2012
Dividend shareholder of record date						Jun. 29, 2012
Quarter 3 [Member]
Class of Stock [Line Items]
Dividend payable date						Oct. 31, 2012
Dividend shareholder of record date						Sep. 28, 2012
Quarter 4 2012 [Member]
Class of Stock [Line Items]
Dividend payable date						Jan. 29, 2013
Dividend shareholder of record date						Dec. 31, 2012
Dividends Payable, Date Declared						Nov. 08, 2012